Depicted in FIG. 1 is an inkjet printer, on which the main parts are labelled as follows: a fixed structure 41, a scanning carriage 42, an encoder 44 and printheads 40 which may be either monochromatic or colour, and variable in number.
The printer may be a stand-alone product, or be part of a photocopier, of a “plotter”, of a facsimile machine, of a machine for the reproduction of photographs and the like. The printing is effected on a physical medium 46, normally consisting of a sheet of paper, or a sheet of plastic, fabric or similar.
Also shown in FIG. 1 are the axes of reference:                x axis: horizontal, i.e. parallel to the scanning direction of the carriage 42; y axis: vertical, i.e. parallel to the direction of motion of the medium 46 during the line feed function; z axis: perpendicular to the x and y axes, i.e. substantially parallel to the direction of emission of the droplets of ink.        
FIG. 2 is an axonometric view of the printhead 40, showing the nozzles 56, generally arranged in two columns parallel to the y axis, and a nozzle plate 106.
The composition and general mode of operation of a printhead according to the thermal type technology, and of the “top-shooter” type in particular, i.e. those that emit the ink droplets in a direction perpendicular to the actuating assembly, are already widely known in the sector art, and will not therefore be discussed in detail herein, this description instead dwelling more fully on some only of the features of the heads and the manufacturing process, of relevance for the purposes of understanding this invention.
The current technological trend in ink jet printheads is to produce a large number of nozzles per head (≧300), a definition of more than 600 dpi (dpi=“dots per inch”), a high working frequency (≧10 kHz) and smaller droplets (≦10 pl) than those produced in earlier technologies.
Requirements such as these are especially important in colour printhead manufacture and make it necessary to produce actuators and hydraulic circuits of increasingly smaller dimensions, greater levels of precision, and narrow assembly tolerances.
These drawbacks are solved, for instance, by means of the monolithic printhead described in the Italian patent application TO 99A 000610, a section of which parallel to the plane z-x is illustrated in FIG. 3, which shows an ejector 55 comprising: a substrate 140 of silicon P, a structural layer 107, one of the nozzles 56; a groove 45; ducts 53; channels 167; and a resistor 27 which, when current passes through it, produces the heat needed to form a vapour bubble 65 which, by expanding rapidly in a chamber 57, results in emission of a droplet of ink 51. Also indicated is a tank 103 containing the ink 142.
Another solution is represented, for example, by a monolithic printhead described in the Italian patent application TO 2000A 000335, shown in sectional view in FIG. 4, which comprises the substrate 140 of silicon P, the structural layer 107, chambers 74 arranged laterally with respect to a lamina 67, on the bottom of which are located the resistors 27, which are therefore external with respect to the lamina 67. Also depicted in the figure are: the groove 45; two pluralities of elementary ducts 75, for each of which only one of the elementary ducts 75 has been drawn, which convey the ink 142 from the groove 45 to the chambers 74; and connecting channels 68. Also shown in the figure is a diameter D which the nozzle 56 presents to the outside of the printhead.
The whole comprising a chamber 74, a nozzle 56, a resistor 27, a connecting channel 68 and a plurality of elementary ducts 75 is called ejector 73.
Both the solutions also comprise a structural layer 107 in which the nozzles 56 are made using known techniques, such as for instance a laser drilling. These techniques have, however, a drawback described in the following: for the head to work properly, it is necessary for the nozzle 56 to have a truncated cone shape with the greater base towards the inside of the head, and the lesser base towards the outside. This is difficult to obtain using the above-mentioned techniques, whereas a nozzle with a truncated cone shape with the greater base towards the outside or, in the best case, a cylindrical shape nozzle is obtained commonly.